Hubble Space Telescope Discovery of a Probable Caustic-Crossing Event in the MACS1149 Galaxy Cluster Field

ATel #9097; Patrick L. Kelly (UCB), Steven Rodney (USC), Jose Maria Diego (Cantabria), Adi Zitrin (Caltech), Tom Broadhurst (Ikerbasque), Jonatan Selsing (DARK), Italo Balestra (INAF), Alberto Molino Benito (IAG/USP), Marusa Bradac (UCD), Larry Bradley (STScI), Gabriel Brammer (STScI), Brad Cenko (GSFC/UMD), Lise Christensen (DARK), Dan Coe (STScI), Alexei V. Filippenko (UCB), Ryan Foley (UIUC), Brenda Frye (UA), Melissa Graham (UW), Or Graur (NYU), Claudio Grillo (DARK), Jens Hjorth (DARK), Andy Howell (UCSB), Mathilde Jauzac (Durham), Saurabh Jha (Rutgers), Nick Kaiser (UH), Ryota Kawamata (Tokyo), Jean-Paul Kneib (Marseille), Jennifer Lotz (STScI), Thomas Matheson (NOAO), Curtis McCully (UCSB), Julian Merten (Oxford), Mario Nonino (INAF), Masamune Oguri (Tokyo), Johan Richard (Lyon), Adam Riess (JHU/STScI), Piero Rosati (Ferrara), Kasper Borello Schmidt (AIP), Keren Sharon (UMich), Nathan Smith (UA), Lou Strolger (STScI), Tommaso Treu (UCLA), Xin Wang (UCLA), Ben Weiner (UA), Liliya Williams (UMN), Weikang Zheng (UCB)
on 29 May 2016; 07:50 UT
Credential Certification: Patrick Kelly (pkelly@astro.berkeley.edu)

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While monitoring the MACS1149 (z = 0.54) galaxy cluster as part of the RefsdalRedux program (PID 14199; PI Kelly) with the Hubble Space Telescope (HST) WFC3 IR camera, we have detected a rising transient that appears to be coincident (<0.1 arcsec) with the cluster's critical curve (line of formally infinite magnification) in cluster models (including those of C. Diego, C. Grillo, M. Oguri, A. Zitrin, M. Jauzac, K. Sharon, and J. Merten). Remarkably, the most likely current interpretation of the transient is that of an individual star in the host galaxy of SN Refsdal (z = 1.49) approaching the cluster caustic owing to the relative motion of the host galaxy, the cluster lens, and Earth. If so, we expect that it is currently magnified by a factor of possibly ten thousand to hundreds of thousands.

Target-of-opportunity optical follow-up imaging in several ACS and WFC3 bands with the FrontierSN program (PID 14208; PI Rodney) has revealed that its rest-frame ultraviolet through optical spectrum may be reasonably well fit with that of a B star at z=1.49 exhibiting a strong Balmer break. At discovery on April 29 UT, the transient had a flux of F125W (J band) = 26.5 +/- 0.1 mag AB, and has subsequently brightened to F125W = 25.8 mag AB and F606W (R band) 26.8 +/- 0.1 mag AB during the most recent visit on May 28 UT. The coordinates we have measured are RA = 11:49:35.66, Dec = 22:23:48.0 J2000 in the CLASH astrometric system.

Our models indicate that the transient, if a caustic-crossing event, will undergo an exponential rise to reach a peak R-band magnitude of ~24 AB or brighter for a period of 5-10 hours, when the magnification may reach of order ten million (see Miralda-Escude 1991), and this should be followed by an extremely rapid decline in flux over a period of several hours. Observations near peak should provide a useful test of the open question of the scale of substructure in the cluster. The current 95% confidence interval spans June 6 and July 7, and favors a date for the peak in mid June.

With the RefsdalRedux and FrontierSN as well as ground-based programs, we are scheduled to monitor the field during the next month to make it possible to anticipate the time of peak for high-cadence ground-based follow-up observations.

We are organizing a coordinated global campaign for follow-up observations near peak brightness in the MACS1149 field, which is only visible during a fraction of the night at any location during June and July. If you have access to a useful observing facility, we would very much welcome your participation in our follow-up effort and encourage you to email our team (pkelly@astro.berkeley and srodney@sc.edu), since a full observation of the light curve will be important for a full interpretation of the event.

An HST image of the transient in the MACS1149 cluster field may be found here.